15. 設 pn表第n個質數，即 p<sub>1</sub> = 2 , p<sub>2</sub> = 3, p<sub>3</sub> = 5 ,L。若&lt; an &gt; 為一數列且&nbsp;<img src="https://img.yamol.tw/item/678179-0-5eb7bfa971f9f.png#s-126,78">，則a5 =？ (A) &nbsp;23/29&nbsp;(B) &nbsp;19/23&nbsp;(C) &nbsp;17/19&nbsp;(D) &nbsp;13/17